In recent years, chemotherapy for hematological malignancies, and high-dose corticosteroid treatment for organ transplant recipients, along with the spread of AIDS, have greatly increased the number of immunocompromised patients (1, 12, 14, 43). Saprophytic filamentous fungi, such as Aspergillus, Rhizopus, and Mucor species, found in the environment and considered to be of low virulence, are now responsible for an increasing number of infections in the immunocompromised host (17, 20, 43). In addition, these infections are often fulminant and rapidly fatal in immunocompromised patients (7, 11, 12, 20, 44). Morbidity and mortality is extremely high; for example, aspergillosis has a mortality rate of approximately 90% (8, 11).
To complicate matters, diagnosis is difficult and symptoms are often non-specific (18, 27, 29, 42, 44). Antibody-based tests can be unreliable due to the depressed or variable immune responses of immunocompromised patents (2, 9, 18, 46). Antigen detection tests developed to date have fallen short of the desired sensitivity (2, 9, 38). Radiographic evidence can be non-specific and inconclusive (5, 29, 36), although some progress in diagnosis has been made with the advent of computerized tomography (40). However, definitive diagnosis still requires either a positive blood or tissue culture or histopathological confirmation (3, 21). An added complication is that the invasive procedures necessary to obtain biopsy materials are often not recommended in thrombocytopenic patient populations (37, 41).
Even when cultures of blood, lung or rhinocerebral tissues are positive, morphological and biochemical identification of filamentous fungi can require several days for adequate growth and sporulation to occur, delaying targeted drug therapy. Some atypical isolates may never sporulate, making identification even more difficult (23). When histopathology is performed on tissue biopsy sections, the morphological similarities of the various filamentous fungi in tissue make differentiation difficult (16). Fluorescent antibody staining of histopathological tissue sections is not specific unless cross-reactive epitopes are absorbed out which can make the resultant antibody reactions weak (14, 19). Therapeutic choices vary (7, 41, 44) making a test to rapidly and specifically identify filamentous fungi urgently needed for the implementation of appropriately targeted therapy. Early and accurate diagnosis and treatment can decrease morbidity and increase the chances for patient survival (6, 27, 39). Furthermore, identification of filamentous fungi to at least the species level would be epidemiologically useful (24, 31, 43, 47).
PCR-based methods of detection, which show promise as rapid, sensitive means to diagnose infections, have been used in the identification of DNA from Candida species (13, 15, 30) and some other fungi, particularly Aspergillus species (31, 33, 45). However, most of these tests are only genus-specific (28, 38) or are directed to detect only single-copy genes (4, 35). Others have designed probes to detect multi-copy genes so as to increase test sensitivity (31, 33) but in doing so have lost test specificity because they have used highly conserved genes, which detect one or a few species but which are also plagued with cross-reactivities to human, fungal or even viral DNA (25, 31, 33).
Therefore, it is an object of the invention to provide improved materials and methods for detecting and differentiating Aspergillus and other filamentous fungal species in the clinical and laboratory settings.